Precambrian crystalline rocks of the Rocky Mountain region of the United States represent two age provinces.An Archean province (older than 2,600 million years) occupies Wyoming and adjacent parts of Utah, Montana, and South Dakota.A Proterozoic province (about 1,600 to 1,800 million years old), is represented by only sparse exposures west and northwest of the older terrane, and by extensive exposures to the south.The Archean province is mostly felsic gneisses and associated metasedimentary rocks that were metamorphosed about 2,800 million years ago.Tonalitic to granodioritic plutons were emplaced in this terrane 2,500 to 2,760 million years ago.In Colorado, a thick sequence of volcanic and sedimentary rocks was deposited between 2,000 and 1, 750 million years ago.These rocks were metamorphosed and intruded by numerous granodioritic plutons about 1, 700 million years ago.This province was invaded by granitic plutons 1,400 million years ago and again, in central Colorado, 1,015 million years ago.Shelf-type sedimentary sequences were deposited on the older crust during the interval from 2,500 to 1, 700 million years ago and are preserved in a belt from southern Wyoming to the Black Hills.A younger sequence, 1,460 to 1,600 million years in age, is preserved only as the Uncompahgre Formation in southwestern Colorado.A still younger sequence, the miogeoclinal Belt Supergroup, 850 to 1,500 million years in age, is preserved in western Montana and northern Idaho.Rocks roughly equivalent to but isolated from the Belt Supergroup include the Y ellowjacket Formation and Lemhi Group of Idaho and the Uinta Mountain Group of northeastern Utah and northwestern Colorado.Eugeoclinal rocks, including diamictites, were deposited west of the miogeoclinal rocks beginning approximately 860 million years ago.
